The present invention is related to photochemically polymerizable liquid crystal compounds, liquid crystalline mixtures which contain such compounds, and the use of the liquid crystal compounds in a cross-linked state as optical or electronic components.
Liquid crystals having at least two photochemically polymerizable groups can be orientated on a substrate or in a cell, for example by orientating layers or in a field. These orientated liquid crystals, provided with a suitable mount of a photoinitiator, are polymerized by irradiation with light of a suitable wavelength. The cross-linked structure which is thereby produced remains even at high temperatures. Such layers can be found, for example as, parts of hybrid layers as described in Swiss Patent Applications CH 2016/94 and CH 2017/94. In this manner, optical components such as retarders, wave guides, optical grids and filters, integrated color filters or cells with piezoelectric and with non-linear optical (NLO) properties can be prepared. These types of optical components can be used, for example, in projection systems.
Further, the requirements for properties such as birefringence, refractive index and transparency which must be fulfilled vary depending on the field of application. For example, networks for optical retarders should have a high birefringence in order to keep the layer thickness of the integrated optical component to a minimum.
Polymers which are electrically insulating, but which have a good thermal conductivity, are used in microelectronics to rapidly and efficiently disperse heat which is formed when a high density of components is operated. There is also a need for insulating layers having a low dielectric constant and a high heat resistance. Homogeneously orientated liquid crystalline networks are advantageous for such applications by virtue of their low thermal expansion coefficients, high thermal conductivity and stability.
In addition to the general interest in photochemically polymerizable liquid crystals for optical and electrical components, there is also a need for such compounds in other areas. These liquid crystalline materials are also suitable for cladding glass fibers for optical data transmission. The use of such networks increases the elastic modulus in the longitudinal axis of the fiber, lessens the thermal expansion coefficient and reduces microdistortion losses. This leads to an increased mechanical stability.
Since liquid crystals are usually used as mixtures of several components, it is important that the components have a good miscibility with one another. Conventional photochemically polymerizable liquid crystals usually have a high melting point and clearing point. However, a spontaneous thermal polymerization can occur prematurely in the processing, which is carried out at temperature barely below the clearing point, because at this temperature the viscosity in the liquid crystalline state is at its lowest and is therefore favorable for a good orientatability. This spontaneous polymerisation leads to the formation of domains, whereby the optical and thermal properties in the cross-linked layers produced can be clearly influenced. The melting point can be decreased by producing complicated mixtures with several components, which indeed permits a processing at lower temperatures, but brings with it the danger of a crystallization of conventional polymerizable liquid crystals.
Therefore, it is an object of the present invention to produce photochemically polymerizable compounds having low melting points and clearing points so that they can be processed very readily at temperatures above room temperature in the liquid crystalline state and also in solution. A further object of the present invention is to produce photochemically polymerizable compounds that can be orientated and structured without forming domains. An additional object of the present invention is to produce polymerizable compounds which are thermally stable and have long term stability in the crosslinked state.